Mesothelioma associated with commercial use of vermiculite containing Libby amphibole

Our plain-English summary of Mesothelioma associated with commercial use of vermiculite containing Libby amphibole, by K.K. Dunning, S. Adjei, L. Levin, et al (Journal of Occupational and Environmental Medicine, 2012).

The study

• Authors: K.K. Dunning, S. Adjei, L. Levin, A.M. Rohs, T. Hilbert, E. Borton, V. Kapil, C. Rice, G.K. Lemasters, J.E. Lockey
• Journal: Journal of Occupational and Environmental Medicine
• Published: 2012;54(11):1359-63
• Cited by: 21 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

Objectives: To describe asbestos-related mortality among manufacturing workers who expanded and processed Libby vermiculite that contained amphibole fiber.

Methods: Standardized mortality ratio was calculated for 465 white male workers 31 years after last Libby vermiculite exposure.

Results: Two workers died from mesothelioma, resulting in a significantly increased standardized mortality ratio of 10.5 (95% confidence interval, 1.3 to 38.0). These workers were in the upper 10th percentile of cumulative fiber exposure, that is, 43.80 and 47.23 fiber-years/cm, respectively. One additional worker with cumulative fiber exposure of 5.73 fiber-years/cm developed mesothelioma but is not deceased. There were no other significantly increased standardized mortality ratios.

Conclusions: Workers expanding and processing Libby vermiculite in a manufacturing setting demonstrated an increased risk for the development of mesothelioma following exposure to the amphibole fiber contained within this vermiculite ore source.

In plain English

This mortality study followed workers at an Ohio plant that expanded and processed vermiculite from Libby, Montana, as a carrier for lawn care products. Libby vermiculite contained asbestiform minerals described as winchite, richterite and tremolite. The analysis covered 465 white men from a 513-worker cohort studied in 1980. They were followed to June 30, 2011; by then 136 had died. Their cumulative fiber exposure was relatively low (median 0.63 fiber-years per cubic centimeter). Two workers died of mesothelioma, giving a standardized mortality ratio of 10.5 (95% confidence interval 1.3 to 38.0) compared with the US population; this ratio counted only deaths from 1999 onward, when mesothelioma got its own disease code. Both were in the top tenth of cumulative exposure (43.80 and 47.23 fiber-years per cubic centimeter). A third worker, with a cumulative fiber exposure of 5.73 fiber-years per cubic centimeter, was diagnosed with mesothelioma in 2010 and was still alive. No other cause of death, including lung cancer, was significantly increased. The authors concluded these workers had an increased risk of mesothelioma. They noted that the small number of deaths limited the study's power.

We wrote this plain-English summary from the full article with the help of AI, and a second AI model checked each sentence against the article. It is not the authors' wording; the abstract and the article are their own account.

What the study did and found

• Design: Retrospective cohort mortality study using standardized mortality ratios (SMRs) compared with the US population, plus standardized rate ratios by exposure tertile
• Who was studied: 465 white men from a cohort of 513 workers at an Ohio manufacturing facility who took part in a 1980 pulmonary morbidity study. Vital status was followed from May 1980 to June 30, 2011. Nonwhite men (16) and women (32) were described but not included in the SMR analysis.
• Exposure: Vermiculite expanded and processed at the plant. Libby, Montana vermiculite, containing asbestiform minerals characterized as winchite, richterite and tremolite, was used from 1959 until 1980. Vermiculite from South Carolina, Palabora (South Africa) and Louisa County (Virginia) was also used. Exposure was measured as cumulative fiber exposure in fiber-years/cc: mean 9.0, median 0.63, range 0.0 to 106.31. The authors note that this fiber measure is not specific to asbestiform minerals.
• Main results:
  • Of the 465 white men, 136 had died as of June 30, 2011.
  • Among white male workers, the SMR for mesothelioma, based on two deaths after 1998, was significantly increased at 10.5 (95% CI, 1.3 to 38.0).
  • The mesothelioma SMR included only deaths from 1999 to June 30, 2011, because a unique code for mesothelioma (ICD-10 C45) was first established in 1999. The authors found no 'possible mesothelioma' deaths before 1999.
  • Both mesothelioma deaths were in the top tenth percentile of cumulative fiber exposure, at 43.80 and 47.23 f-yr/cc.
  • One additional worker, with cumulative fiber exposure of 5.73 f-yr/cc, was diagnosed with mesothelioma in 2010 at age 66 and was not deceased at the time of the report.
  • Among white male workers there were no other significantly increased SMRs. Lung cancer: 16 observed deaths versus 16.9 expected, SMR 0.9 (0.5 to 1.5). Asbestosis: 1 death, SMR 15.4 (0.4 to 85.9).
  • Among white male workers, the all-cause SMR was significantly low at 0.8 (95% CI, 0.6 to 0.9), which the authors interpreted as suggesting a healthy worker effect.
  • For cancer of the digestive system and peritoneum, standardized rate ratios rose with increasing exposure category (3.4 and 4.2 versus the lowest tertile), but were not statistically significant.
• The authors' conclusion: The authors concluded that workers expanding and processing Libby vermiculite in a manufacturing setting showed an increased risk of mesothelioma following exposure to the amphibole fiber in this vermiculite. They added that further follow-up of the aging cohort would provide more data and statistical power.
• Limitations the authors state:
  • The number of deaths was relatively low, reducing the power to detect mortality related to Libby vermiculite exposure.
  • Exposure estimates derived from historic industrial hygiene records may have been misclassified.
  • The cumulative fiber exposure measure is not specific to asbestiform minerals and may include other fibers.
  • The low all-cause mortality suggests a healthy worker effect.
  • SMRs by exposure category should be interpreted with caution because person-years may be distributed differently by worker age.
  • There were too few nonwhite and female workers for SMR analysis by race or gender.

Mines named here

Asbestos mines this study is about. Each mine's page says who ran it, when it worked, and what it produced.

• Libby vermiculite mine, Montana

All asbestos mines

Topics

The National Library of Medicine indexes this study under subjects our site covers: Lung cancer, Mesothelioma, Talc and vermiculite.

Medical terms in this study, explained in our Lexicon: Mesothelioma, vermiculite, Libby amphiboles, Libby Vermiculite, Amphibole Group, standardized mortality ratio (SMR), confidence interval, Fiber Year.

Related studies

• Low-level fiber-induced radiographic changes caused by Libby vermiculite: a 25-year follow-up study — A.M. Rohs, J.E. Lockey, et al. (2008)
• Exposure to asbestos-containing vermiculite ore and respiratory symptoms among individuals who were children while the mine was active in Libby, Montana — L.C. Vinikoor, T.C. Larson, et al. (2010)
• Mortality in a cohort of vermiculite miners exposed to fibrous amphibole in Libby, Montana — J.C. McDonald, J. Harris, et al. (2004)
• Vermiculite worker mortality: estimated effects of occupational exposure to Libby amphibole — T.C. Larson, V.C. Antao, et al. (2010)
• Asbestos-related radiographic findings among household contacts of workers exposed to Libby vermiculite: impact of workers' personal hygiene practices — T.J. Hilbert, A. Franzblau, et al. (2013)
• Radiographic abnormalities and exposure to asbestos-contaminated vermiculite in the community of Libby, Montana, USA — L.A. Peipins, M. Lewin, et al. (2003)

Browse more research in Journal of Occupational and Environmental Medicine, or return to medical research.